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arXiv · 1804.02189

Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts

Frank R. N. Chambers·Anna L. Watts·Yuri Cavecchi·F. Garcia·L. Keek
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Abstract

Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The frequencies can be offset from the spin frequency of the NS by several Hz, and can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean, the frequency of which would decrease (in the rotating frame) as the burst cools, hence explaining the drifts. Most burst oscillations have been observed during H/He triggered bursts, however there has been one observation of oscillations during a superburst; hours' long Type I X-ray bursts caused by unstable carbon burning deeper in the ocean. This paper calculates the frequency evolution of an oceanic r-mode during a superburst. The rotating frame frequency varies during the burst from 4-14 Hz, and is sensitive to the background parameters, in particular the temperature of the ocean and ignition depth. This calculation is compared to the superburst oscillations observed on 4U-1636-536. The predicted mode frequencies ($\sim$ 10 Hz) would require a spin frequency of $\sim$ 592 Hz to match observations; 6 Hz higher than the spin inferred from an oceanic r-mode model for the H/He triggered burst oscillations. This model also over-predicts the frequency drift during the superburst by 90 %.

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Frank R. N. Chambers, Anna L. Watts, Yuri Cavecchi, F. Garcia, L. Keek. 2018-04-06. Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts. https://doi.org/10.1093/mnras%2Fsty895

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